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# Is 1,5 meters the optimum span for a DLG?

Posted by l shems | Dec 08, 2017 @ 01:00 PM | 4,586 Views
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O previously posted some analysis on DLG design in this Blog. After some discussions about smaller than 1.5 meter wing span DLG's possibly outperforming the full size ones on the last WC in the Ukraine, I dug up my old analysis, and worked on it a little to find out if I could derive the theoretical optimum span for a DLG. That is to say: optimum for maximum airtime in still air at a fixed launch speed.

It turns out that actually the current launch speed determines the optimum terminal velocity to design the plane for. So making the plane faster or slower with respect to this terminal velocity would lower the total airtime for this launch speed. A terminal velocity of only 60% of the launch speed would be the optimum design speed for obtaining maximum airtime.

Secondly, the maximum airtime itself can be expressed in the L/D ratio and the launch speed only! So, basically speaking, just optimise L/D and you will optimise maximum airtime (for the optimum design speed that is). The expression is quite simple:

t_(flight,max) ≈ 2 / 3g ∙ (L/D)_optimum ∙ √((L/D)_optimum ) ∙ v_launch

When entering an L/D of 12 to 20, and a v_launch of 40 m/s, the still air time will be between 115 and 245, with an average 180 seconds.

Finally, the question I started with. The expression for terminal velocity that you...Continue Reading

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